1. Field of the Invention
The present invention is directed to distortion measurement of glass substrates.
2. Technical Background
A liquid crystal display (LCD) is a thin flat display device used in conventional television, cellular telephone, computers, etc. Conventional LCD manufacturing techniques are complex and require a variety of integrated technologies. The LCD includes a cover glass substrate and a back glass substrate. The back substrate often includes electronics, such as thin-film transistors (TFTs) that must be aligned between the cover substrate and the back substrate. TFT technology has advanced to the point where each TFT is implemented in a sub-micron area within the LCD. As such, sub-micron accuracy is required when aligning TFTs within the substrates.
During manufacturing a variety of manufacturing operations may uniformly or non-uniformly distort an LCD display. For example, heat treatment often causes compaction and/or distortion. The compaction may be uniform, but the compaction may also be non-uniform. Knowledge of how heat treatment effects the substrate is critical to properly aligning TFTs in an LCD. Therefore, it is necessary to measure compaction and distortion to properly position the TFTs and align the back substrate, the TFT, and the cover substrate.
A variety of conventional stress measurement, compaction measurement, and distortion measurement technologies such as measurement gauges are available for measuring the effects of processing such as heat treatment on substrates. Many of these measurement technologies include a support table with reference markings that are used to inscribe the substrate relative to the reference markings. The substrate is positioned on the support table and inscribed using the reference markings as a guide. A process such as heat treating, cutting, etc is then applied to the substrate. The substrate is then repositioned on the support table and the markings on the substrate are compared to the reference markings on the table to determine changes. Typically a visual system is employed to distinguish the very small changes.
Compaction measurement may be performed on a sample substrate to calibrate conventional measurement technology such as a distortion measurement gauge. Compaction measurement includes measuring the location of grid points inscribed on a sample substrate relative to reference points (i.e., measured value), processing the sample substrate, re-measuring the location of the grid points relative to the reference points (processed value), and determining the difference between the pre-processed and the post-processed grid point locations. The process value is compared against the measured value to determine the accuracy of the distortion measurement. Specifically, the difference between the two grid points is an indication of the amount of compaction or distortion that occurred on the substrate. The compaction or distortion measurements are then used to calibrate the measurement gauge.
A number of issues occur during processing and inhibit operator ability to calibrate a test and measurement system at the level of accuracy required. For example, in the case of heat treatment with a furnace, the accuracy of the measured value is based on three components. The first component is the measurement gauge. The second component is the temperature fluctuations in the furnace, and the third is the variation in the sample substrate. These factors preclude measuring distortion to the required accuracy needed for alignment of modern TFT technology. As a further note, the sample can only be used once. Therefore, when calibrating or determining the accuracy of a measurement gauge, cost and the availability of samples can become an issue.
Thus there is a need for determining the accuracy of a measurement and inspection gauge. There is a need to calibrate a measurement and inspection gauge at the levels of accuracy and with the levels of repeatability required of modern LCD displays that include electronics such as TFTs.